Humpback whales (Megaptera novaeangliae) attend both Mexico and Hawaii breeding grounds in the same winter: mixing in the northeast Pacific

Humpback whales that assemble on winter breeding grounds in Mexico and Hawaii have been presumed to be, at least, seasonally isolated. Recently, these assemblies were declared Distinct Population Segments under the US Endangered Species Act. We report two humpback whales attending both breeding grounds in the same season—one moving from Hawaii to Mexico and the other from Mexico to Hawaii. The first was photo-identified in Maui, Hawaii on 23 February 2006 and again, after 53 days and 4545 km, on 17 April 2006 in the Revillagigedo Archipelago, Mexico. The second was photo-identified off Guerrero, Mexico on 16 February 2018 and again, 49 days and 5944 km later, on 6 April 2018 off Maui. The 2006 whale was identified in summer off Kodiak Island, Alaska; the 2018 whale off British Columbia. These Mexico–Hawaii identifications provide definitive evidence that whales in these two winter assemblies may mix during one winter season. This, combined with other lines of evidence on Mexico–Hawaii mixing, including interchange of individuals year to year, long-term similarity of everchanging songs, one earlier same-season travel record, and detection of humpback whales mid-ocean between these locations in winter, suggests reassessment of the ‘distinctiveness' of these populations may be warranted.

Largely owing to distance of separation, it has been presumed that the Mexico and Hawaii winter assemblies are, at least, seasonally isolated. That is, whales leave northern feeding grounds and migrate to one winter assembly or the other, then after breeding success or seasonal residency, return to feeding areas. In 2016, the US National Marine Fisheries Service (NMFS) went significantly further, to designate the Mexico and Hawaii assemblies as Distinct Population Segments (DPS). 1 The whales within each region were given different conservation status under the Endangered Species Act: Mexico 'Threatened' and Hawaii 'Not at Risk'-inferring biologically separate entities [6,7].
This paradigm of separate humpback whale populations in the North Pacific emerged in the 1990s and 2000s. Regional and Pacific-wide photo-identification sampling indicated strong migratory preferences between specific feeding grounds and breeding areas, for example, Southeast Alaska and Hawaii or Pacific Northwest and Mexico [8][9][10][11]. Genetic studies found differences in haplotype frequency between the photo-ID sampled areas, including between the breeding grounds [12]. Both ID matching and genetic studies recognized 'exceptions to these (migratory) patterns' and 'potential high levels of plasticity in (whale) movements'. However, the generality became the basis of the DPS designations [6].
In fact, there are multiple lines of evidence of mixing between Mexico and Hawaii humpback whale populations. Individual whales interchange between Hawaii and Mexico from one winter to the next (e.g. [8][9][10][11][13][14][15]). Hawaii and Mexico populations share some and often all of the same phrases in their complex and changing song-something impossible without mixing at some point in annual cycles [16][17][18][19]. Whales from multiple feeding areas mix in one breeding area, (e.g. whales from Alaska, British Columbia and Russia in Hawaii); whales in a single feeding area may migrate to different breeding areas (e.g. whales from Russia found in Asia, Hawaii, Mexico) [9,13,14,[20][21][22]. Pacific-wide song comparisons point to ocean basin scale interactions, not divisions [19]. A same winter match was reported, an individual whale being identified in the winter of 1986 in both Mexico (February) and Hawaii (April) [23]. Humpback whales have been detected acoustically within breeding seasons at latitudes midway between Mexico and Hawaii [24].
Here, we provide further direct evidence of mixing between Mexico and Hawaii humpback whale assemblies with the report of two more individually photo-identified whales that travelled between these regions in the same winter-that is, attended breeding assemblies in both the eastern and central North Pacific within a single breeding season.

Methods (a) Photo-identification
The photo-identification of individual whales by the unique and permanent skin pigment patterns on the underside of the flukes has, since the 1970s [25], become the basis for the majority of studies of humpback whale abundance, migrations, population definition and behaviour. Recently, very accurate recognition software and computerized matching programs have enabled large-scale comparison of tens of thousands of identifications and provide an increasingly detailed picture of humpback whale movement patterns in the North Pacific and worldwide [26]. This report is a result of Happywhale (www.happywhale. com) computerized matching efforts.

(b) Effort
Overall, in its entire North Pacific matching project, Happywhale compared 26 607 humpback whale individuals from years 1977 to 2020 from 1851 contributors from locations spanning the ocean, including Mexico, Hawaii, Japan, the Philippines, California, Oregon, Washington, British Columbia, Alaska and Russia. Contributors of photo-identifications included long-term research projects, whale watch tour operators and the general public.
This effort generated multiple sightings of the two individual whales pertinent to this report, listed in table 1 and summarized in figure 1. One individual was documented four times from 2004 to 2006 in three locations: Hawaii [2]; Mexico [1]; Kodiak Island, Alaska [1]. The second whale was identified 15 times between 2004 and 2021, in four locations: Hawaii [3]; Mexico [1]; northern British Columbia [1]; southern Bristish Columbia/ northern Washington [11]. Of these 19 sightings (both cases combined), seven came from tour operators and 12 from research projects.

Results
The sightings histories of the two whales that travelled between Mexico and Hawaii in one winter are listed in table 1 and illustrated in figure 1. Figure 2 shows the photographic identification matches.    Then, on 6 April 2018, 49 days later and 5944 km distant, this whale was identified in the Auau Channel off West Maui near Olowalu. It was one of seven whales pursuing a female in a surface-active group and very likely a male (more than one female in one of these groups does occur but is rare). It was observed for 40 min (14.30-15.10 HST) with no indication that it was the PE. It was one of the secondary escorts or challengers in the group. This was the only identification of this whale in Hawaii that season. This whale's age, assuming it was at least a yearling in 2004 (that is, it was not identified then in a mother-calf pair), would be, at the time of the Mexico-Hawaii match, a minimum of 15 years old, and it was likely sexually mature [27].

(c) Travel times
It is not possible to determine actual travel times since we cannot know date of departure from one assembly area or date of arrival in the other. Nor can we know if travel was direct and steady or if whales lingered at some point between the departure and destination. However, rough calculations can be made which suggest a range of travel times as shown in table 2. It is unlikely that whales were photo-identified the day they departed and the day they arrived so the ID to ID are likely overestimates of travel time (in days).

Discussion
Individual humpback whales (at least males) may travel between the distant (4500-6000 km apart) Mexico and Hawaii breeding assemblies in the same winter season. These travel records are consistent with the recent, midocean detection of humpback whale songs in winter between the locations [24]. In fact, this mid-ocean detection occurred in 2018, the year of the Guerrero-Hawaii travel. The delineation of these traditional winter grounds has become less clear in that not only may humpback whales attend both regions during a winter breeding season, they may also be present over a broad reach of the tropical North Pacific, between Hawaii and Mexico at that time.
The direct, within-season travel records between Mexico and Hawaii bolster 2 an earlier, similar report [20]-and provide two new insights. The first is that, not only do whales travel east to west, from Mexico to Hawaii, but they also travel west to east, Hawaii to Mexico, in one season. The second insight is that not only do whales travel between Hawaii and the westernmost Mexico breeding habitat at Isla Clarión in the Revillagigedo Archipelago, but also between Hawaii and southern mainland Mexico, some 1000 km further distance.
We are not aware of other examples of same-season connectivity between humpback whale breeding grounds isolated by large longitudinal distances. However, several instances of same-season movement between breeding assembles have been reported in the southwestern Pacific [34]. Direct comparison of this behaviour between the northeast and southwest Pacific is complicated by the relatively contiguous island chains (that is, breeding habitat) in the south versus the 4500-6000 km of deep ocean between Mexico and Hawaii in the north. Nonetheless, the South Pacific observations do indicate that humpback whales may attend more than one geographically defined breeding ground in one season.
Beyond the Mexico-Hawaii connections are the multiple summer sightings of the 2018 Match 2 whale (in 7 of the 18 years from 2004 to 2021) off British Columbia, with sightings in six of those years in the same locale off the southwest coast of Vancouver Island. The location of this whale in the other 11 years is not known. Notably, this whale was not identified in the summer that followed the Hawaii 2017 sighting, or in the summer following the 2018 Mexico-Hawaii match, but was again found in this Vancouver Island location in summer 2019, 2020 and 2021. Humpback whales identified in this one summer feeding ground off southwest Vancouver Island (along less than 200 km of coast) have been found in all four DPS designated breeding grounds: Japan, Hawaii and Mexico, and Central America [10,11,14,21,22] and in the case of this report, two of these grounds in one season. These observations support the hypothesis of a level of fidelity to specific feeding grounds [12,35], but, at the same time, suggest there is potential for widespread mixing during the winter breeding season.
The single record of the 2006 Match 1 whale in a feeding ground off Kodiak Island, Alaska indicates that it is not only whales from a specific feeding area off British Columbia that may attend Hawaii and Mexico breeding grounds in one winter.
The same-season Mexico-Hawaii travels equate with observations of song sharing [16][17][18][19], and interchange of individual whales year to year (e.g. [9][10][11]13]). Together these studies indicate decades-long interaction between whales that use these two winter regions. Consistent with this view are the observations of whales from one feeding area migrating to both (and more) breeding locations, where they mix with whales from other feeding grounds [13,14,21,22]. This collective evidence would lead logically to a hypothesis of one panmictic, or several highly overlapping, humpback whale populations in the northeast Pacific-something that was initially proposed in the late 1970s [17].
While the application of the DPS designations may serve an important role for the US Endangered Species Act, real questions arise as to whether these designations, in current form, are a useful reflection of the biology of North Pacific humpback whales. In the formation of these DPS, a number of factors were not given weight, or even considered: multiple breeding ground destinations from one feeding ground; mixing of whales from multiple feeding grounds in one breeding ground; year to year interchange between breeding grounds; and song sharing. Further, the data most influential in DPS designation, the genetic comparisons [12], may be open to question. They were based on the 'a priori' determination of the groups to be compared, known to result in the recognition of artificial genetic differentiation between groups when none may actually exist 3 [36][37][38][39][40].
Evidence of mixing between the whales that compose the Mexico and Hawaii populations is indisputable; the question now is one of significance. Is the mixing a rare occurrence with negligible biological impact or management consequence, or is it a reflection of a biologically meaningful integration of humpback whales throughout the northeast Pacific-if not the entire ocean basin?
Data accessibility. The data on which this paper is based is accessible at https://happywhale.com/individual/15116, and https://happy whale.com/individual/7270. The key data is all included in the main body of the paper. These are the photo-ID photographs in figure 2. Also, figure 1 and table 1 list other ( peripheral) sightings (and photo-identifications) of the same whales. The actual identification photographs of the peripheral sightings, along with date and location information, are available at the links above. The ID photographs are contributed to www. happywhale.com by research groups and whale watch companies. The ID photograph comparisons, leading to the sightings history of individual whales, are made with a computer matching program then checked visually. As shown in the examples in the paper the matches are clear, that is, there is enough information on the identification photographs to be certain they are the same whales. This is a standard technique used in whale research programs worldwide. A detailed description of the computerized matching process in given in [26].
Authors' contributions. J.D.: investigation, resources, visualization, writing-original draft and writing-review and editing; K.A.: data curation, investigation, resources, visualization and writing-review Table 2. Estimates of Hawaii-Mexico travel times. Speeds of travel used in the calculations came from: (1) satellite tags in the North Pacific 4.5-6.2 km h −1 [28] and an average of 4 km h −1 [29], in the South Pacific 3.53 ±2.22 km h −1 [30], and in the South Atlantic the fastest speed recorded at 7 km h −1 [31]; (2) a North Pacific migration photo-identification match [32] indicating 4.79 km h −1 the minimum speed; and (3) the measurement of migratory speed off eastern Australia [33], a range depending on behaviour but with a mean of 4 km h −1 .  [23]. and editing; T.C.: data curation, investigation, resources, visualization and writing-review and editing; B.G.: data curation, investigation, resources, visualization and writing-review and editing; E.L.: data curation, investigation, resources, visualization and writing-review and editing; J.U.: data curation, investigation, resources, visualization and writing-review and editing. All authors gave final approval for publication and agreed to be held accountable for the work performed therein.